1. Field of the Invention
The present invention relates to a method of forming an organic thin film by stacking monomolecular layers using the Langmuir-Blodgett's technique, and, more particularly, to a method of forming an organic thin film having a uniform Y-type stacking structure.
2. Description of the Related Art [Recently, organic thin films (LB film) formed by the Langmuir-Blodgett's technique have been widely studied for the purpose of applications to various types of devices with new functions. If the LB film is used, a device having a desired function may be obtained by controlling the orientation, stacking structure, and inter-molecular distance of functional molecules having a dyestuff skeleton within the film.
In general, the LB film can be obtained in the following manner. At first, amphiphatic organic molecules, each having a hydrophilic portion and a hydrophobic portion is developed over the water surface. The area of the water surface on with the molecules have been developed is then reduced so that a predetermined surface pressure can be obtained, thereby forming a monomolecular layer on the water surface. Then, the monomolecular layer is taken up on a given substrate. Methods of taking up the monomolecular layer on the substrate includes a vertical dipping method, wherein the substrate is vertically dipped in the water surface over which the monomolecular layer is developed, and a horizontal adhesion method, wherein the substrate is adhered to the monomolecular layer and is then pulled up, while the substrate is kept in parallel to the water surface. In forming the LB film, it is necessary to the surface pressure of the monomolecular layer be constantly kept at a predetermined value or above. In a conventional LB film forming apparatus, when the substrate is pulled up, the lowering of the surface pressure due to the removal of the monomolecular layer portion on the water surface near the substrate is immediately transmitted to the entire monomolecular layer. The lowering in surface pressure must be detected by a surface pressure measuring device arranged at a given position in a water reservoir, and a feedback operation must be performed so as to keep the surface pressure at a constant value. The inventors of the present invention, however, has found that this condition can be satisfied only when limited types of aliphatic molecules are used. For example, according to the inventors' study, in the case of dye-containing molecules or high polymers, the viscoelasticity property of the monomolecular layer is high, and the lowering of surface, pressure in the vicinity of the substrate is not immediately transmitted to the surface pressure measuring device arranged remote from the substrate. It is thus difficult to keep a constant surface pressure in stacking monomolecular layers of dye-containing molecules or high polymers by using, for example, the vertical dipping method. Namely, the density and molecular orientation of the monomolecular layer are changed, resulting in the formation of a non-uniform, defective stacked LB film.
Considering prior studies relating to the horizontal adhesion method, an excellent stacked film has not been obtained by this method, although in these studies the horizontal adhesion method were applied mainly to the formation of a polymerized film. It is generally considered that the stacking structure of the stacked film formed by the horizontal adhesion method is of the X type or Z type. In the X type or Z type stacking structure, the molecular orientation of an adhered layer and that of the next adhered layer are identical. In contrast, in the Y type stacking structure, hydrophilic radicals attract one another and hydrophobic radicals attract one another, that is, monomolecular layers are stacked with the molecular orientations of thereof being reversed alternately. In the case where a Y type stacked film can be formed by using amphiphatic molecules, such a stacked film, in fact, has not been obtained in the horizontal adhesion method. The reason for this seems to be that a step of pulling up the substrate is not well controlled, and, even if the first layer is adhered with no problem, the water surface pressure in the vicinity of the substrate is lowered, and the structure of the second layer is deteriorated.
In order to prevent the deterioration in structure of the second layer in the case of the horizontal adhesion method, it is proposed to provide a partition wall surrounding the substrate to prevent the monomolecular layer outside the partition wall from flowing into within the partition wall, and the substrate is pulled up. However, in this case, since the hydrophilic radicals of the monomolecular layer attached on the pulled substrate appear on the surface side a great deal of water is attached to the hydrophilic radicals. This results in disturbance in structure of the next adhered film. Thus, a stacked film with high quality cannot be obtained.
As stated above, regarding the LB film stacking method using the conventional horizontal adhesion method, the stacking mechanism itself has not been fully understood, and therefore the control in the stacking process is not sufficient. Thus, an LB film with high quality has not been obtained.